Method for spent potliner processing separating and recycling the products therefrom

ABSTRACT

A method is provided for efficient processing of spent potliner, separating and recycling/utilizing the liquid and solid products generated during processing. The method includes the steps of crushing and screening the potliner freed from metal debris, pulverizing the potliner suspended in a hot aqueous slurry by autogenous grinding in a tumbler in the presence of air, subjecting the potliner to a series of leaching, thickening and filtering steps followed by separation of the potliner into Bayer Liquor, solid sodium fluoride, quality carbon product and mineral matter characterized by low cyanide and fluoride leachability. About 85% of the water required for potliner processing is recovered and recycled into the process.

[0001] The present invention is directed to a method for efficient processing of spent potliner, separating and recycling/utilizing the liquid and solid products generated by the process.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] Production of aluminum generates 15-55 kg of spent potlining material (from electrolysis cells) per ton of aluminum which amounts up to 1,000,000 tons annually worldwide (1, 2). Spent potlining material has been designated in some countries as hazardous waste because of its high contents of fluorides and cyanides which can leach out and result in major contamination of the groundwater. There is a need for a technology that can process the spent potliner, destroy leachable cyanides, recover valuable chemicals and carbon from the potliner, recycle/utilize residual mineral matter and accomplish that at reasonable capital and processing costs.

[0004] 2. Description of Prior Art

[0005] Because spent potliner has been declared in some countries as hazardous waste, some primary aluminum producers are being faced with regulations that prescribe how the potlining waste must be treated prior to landfilling.

[0006] The processes developed for potlining waste treatment can be classified as thermal treatment processes, gas treatment processes and low temperature leaching processes (2).

[0007] Thermal treatment processes are carried out at high and very high temperatures and usually require solid additives (like dolomite, silica sand) that convert the spent potliner into glass-like, non-hazardous, low leachability waste for which the volume is typically twice larger compared to the volume of original spent potliner. The Ormet process (3) is considered to be one of the best thermal treatment processes though its capital and processing costs are very high.

[0008] Gas treatment processes are carried out in the presence of a specific gas at moderate to high (400-900° C.) temperatures. None of the gas treatment processes is commercially used.

[0009] Many low temperature leaching processes have been developed and most of them, like Alcan's (4,5) Low Caustic Leaching and Liming (LCLL) were derived from cryolite (Na₃AlF₆) recovery process based on leaching the cryolite, pulverized to below 28 Tyler mesh, with aqueous low concentration solution (10-60 g/L) of caustic soda (NaOH). Such leaching can quantitatively decompose the cryolite (6) and, when applied to potliner, can extract a variety of compounds including sodium fluoride and some cyanides into the leach liquor. From 20-40 w/w % of original spent potliner can be converted into leach liquor. The LCLL process decomposes the cyanides contained in the leach liquor by hydrolysis of cyanides. Prior to hydrolysis, the liquor has to be freed of particulates. The hydrolysis takes place in a tubular reactor that operates at pressures of about 200 psi, temperatures 180-250° C. and a residence time of about 30-60 min. None of the low temperature leaching processes, including the LCLL process, deals effectively with the remaining 60-80 w/w % of the residue left after leaching.

[0010] It is the object of the present invention to provide a method for effective autogenous grinding of spent potliner suspended in an aqueous slurry containing caustic soda in concentrations 0-10 g/L or 60-200 g/L, followed by digestion of pulverized particles of the potliner, recovery of undissolved metallic aluminum, carbon catalyzed oxidative conversion of solubilized cyanides to benign formates, solubilization of various chemical compounds formed and separation of some of these compounds to form Bayer Liquor for recycling within the aluminum manufacturing process.

[0011] It is another object of the present invention to provide a method for separation of a quality carbon fuel from the leached residues with acceptable ash content and low fluorine and sodium contents and to recycle/utilize the mineral matter obtained after carbon separation as a substrate for, for instance, forming a new lining for electrolysis cells. The present invention therefore enables total recycling of the spent potliner and eliminates the need for landfilling process residuals.

[0012] These and other objects of the present invention will be apparent from the following description of the preferred embodiments, and the appended claims and from practice of the invention.

SUMMARY

[0013] The present invention provides an efficient method for autogenous grinding and processing of resultant pulverized spent potliner suspended in aqueous solution of caustic soda at selected concentrations, separating the processed potliner into product streams using a system specifically designed for this purpose and recycling/utilizing these product streams; the method includes the steps of crushing and screening (e.g. at 100 mm) the spent potliner freed from metallic aluminum and steel/iron debris, pulverizing the spent potliner by tumbling it in a hot aqueous slurry in the presence of air (oxidation of cyanides), screening the slurry (e.g. at 0.5 mm), recycling the oversize particles for tumbling, extracting/leaching the pulverized particles (e.g. below 0.5 mm) of the spent potliner suspended in diluted aqueous slurry, completing oxidation of solubilized cyanides, thickening the slurry through removal of clarified leachate, diluting the thickened slurry with recycled/distilled water and continuing the extraction process, thickening the slurry again, filtering the thickened slurry and washing the solid particles with hot recycled/distilled water, suspending the washed particles again in recycled/distilled water followed by repeating the extraction process; thickening the slurry and separating it by size, density and flotation based techniques into mineral matter and carbon fuel products, collecting all leachates and filtrates and treating them in a commercial water treatment and distillation system that generates Bayer Liquor, solid sodium fluoride and recycled/distilled water required for spent potliner processing.

[0014] The present invention, as summarized above, separates and recovers metallic aluminum, high calorific value carbon fuel product, low-leachability mineral matter product and Bayer Liquor. Cyanides content in Bayer Liquor is less than what is environmentally acceptable. The Bayer Liquor contains precipitated sodium fluoride (NaF) that can be filtered off and utilized in an aluminum manufacturing process; the remaining Bayer Liquor is a valuable feed for Bayer plant.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In the accompanying figures:

[0016]FIG. 1 presents a diagram of a process for treating spent potliner and generating useful/recycleable products therefrom.

[0017]FIG. 2 presents a flowsheet of a specific extensive extraction and cyanides oxidation system, which is an integral part of a process for treating spent potliner and generating useful/recycleable products therefrom.

[0018]FIG. 3 presents a mixer for direct heating with steam and extracting/leaching soluble components from pulverized spent potliner suspended in aqueous slurry.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The method, according to the present invention, is directed towards autogenous grinding of the crushed and prescreened potlining material suspended in aqueous solution of caustic soda, recovery of residual metallic aluminum, efficient digestion of the potlining material followed by extracting/leaching the soluble components of the potliner, carbon catalysed oxidative destruction of the cyanides in the presence of atmospheric oxygen, separation of leached/extracted components for utilization as Bayer Liquor and solid sodium fluoride, separation and utilization of carbon fuel product and recovery/utilization of the carbon free mineral matter product as it is shown step by step in FIG. 1.

[0020] After three to eight years of operating the Hall-Heroult electrolysis cells, the refractory lining and carbon based components of the cells have to be removed, due to stress, fractures, erosion and chemical reactions. The removed material known as spent potliner is composed of carbon, metallic aluminum, steel/iron debris, a variety of sodium and fluorine derivatives, aluminosilicates, silicates, alumina, aluminates, cryolite, sodium carbonate, metallic sodium, free and complex cyanides and other compounds. The elemental composition of the spent potliner is usually within the following ranges: Constituent Composition range (w/w %) Carbon 10-51 Sodium  7-20 Aluminum  5-22 Fluorine  6-19 Silicon  0-12 Calcium 1-3 Lithium 0.3-1.1 Magnesium 0.3-0.9 Iron 0.0-2.1 Sulfur 0.1-0.3

[0021] The chemical components that render the spent potliner hazardous are cyanides, sodium fluoride and to a lesser degree arsenic and polycyclic aromatic hydrocarbons (PAH). Heavy metals occur at very low concentrations and do not pose any environmental problems.

[0022] To prepare the spent potlining material for processing, according to the present invention, the oversize material (e.g. over 100 mm) must be freed of structural metal and debris, by magnetic separation of iron/steel and manual removal of aluminum, and subjected to preliminary crushing to reduce the size of individual particles to, for instance, less than 100 mm. The so prepared spent potlining material is fed, at a constant rate, into a tumbler which is also supplied with predetermined quantities of solid or liquid caustic soda (NaOH), make-up water and air. The tumbler is equipped with lifters which enhance the tumbling action and effective shear/agitation thus resulting in autogenous grinding of the feed potliner. The exothermic reactions between the aqueous solution of caustic soda and the pulverized particles of spent potliner maintain the temperature in the tumbler at close to the slurry boiling point. The air supplied to the tumbler provides oxygen required for carbon catalyzed oxidative destruction of solubilized cyanides in high pH aqueous solutions. The rate of oxidative destruction can be further enhanced, if required, by providing minute (ppm) quantities of copper to promote catalytic effects of carbon (7).

[0023] The hot slurry exiting the tumbler (FIG. 1) containing about 50-65 w/w % solids (mineral and carbon particles) ranging in size from a few microns to, for instance, less than 100 mm, is subjected to trommel screening. The oversize particles (usually above 0.5 mm) are washed with recycled/distilled water, inspected for metallic aluminum which is manually removed, and then either subjected to size reduction and recycled for tumbling or recycled for tumbling with no reduction in size.

[0024] The slurry containing undersize (usually 0.5 mm and less) particles is diluted with recycled/distilled water, to lower the solids concentration in the slurry to about 15-30 w/w % and the hot fine slurry enters the extensive extraction and cyanides oxidation system. The extensive extraction and cyanides oxidation system is composed of two or more batteries of extracting/leaching mixers (each battery has one or more extracting/leaching mixers), one or more thickeners (or other thickening devices) and at least one filtering device. A specific extensive extraction and cyanides oxidation system is shown in FIG. 2. A single slurry heating and extracting/leaching mixer is shown in FIG. 3.

[0025] In an extensive extraction and cyanides oxidation system, as shown in FIG. 2, the hot fine slurry passes through first battery of three extracting/leaching mixers, supplied from the bottom with steam and air. The cyanides oxidation is completed in the first battery and extensive extraction/leaching of sodium fluoride commences. The hot fine slurry enters thickener #1, where the particles are thickened at the bottom to about 40-50 w/w % solids concentration. The thickened slurry is pumped out, diluted with recycled/distilled water and directed to the second battery of extracting/leaching mixers heated with steam. In the second battery of mixers the extraction and leaching of sodium fluoride intensifies due to reduced concentration of caustic soda, aluminates and other readily solubilized components of the digested spent potliner. The slurry exiting the second battery of mixers is thickened in thickener #2. The thickened slurry (about 40-50 w/w/ % solids) is directed to a vacuum filter belt where the separated/filtered particles are thoroughly rinsed with hot recycled/distilled water and dewatered. The dewatered particles exiting the filter belt are converted into slurry using recycled/distilled water and the slurry is passed through the third battery of extracting/leaching mixers heated with steam. Under these conditions of extensive extraction the residual sodium fluoride present in the particles of digested spent potliner leaches out.

[0026] The primary hot fine slurry exiting the extensive extraction and cyanide oxidation system (FIG. 2) is thickened (FIG. 1) to about 40-50 w/w % solids concentration, and the thickened slurry is subjected to separation using spiral separators, into two slurry streams; the first stream containing, for instance, mineral particles of about 0.05 mm and larger; the second stream containing all carbon particles and mineral particles of about 0.05 mm and smaller.

[0027] The slurry stream containing carbon particles and mineral particles of about 0.05 mm and smaller, having total solids concentration of about 15-30 w/w %, is vigorously agitated in a high shear agitator in the presence of a suitable mineral oil or suitable blend of mineral oils (the amount of oil required is about 0.1-10.0 w/w % based on total mass of carbon particles present in the slurry). The intensity of agitation is determined by the amount of energy required to emulsify the oil in the slurry. Alternatively, the mineral oil(s) can be first emulsified with small quantities of water (e.g. 1 part oil/3 parts water), using any commercially available emulsification system, prior to admixing with slurry stream containing carbon particles and mineral particles of about 0.05 mm and smaller. The emulsified oil is selectively adsorbed on the surface of carbon particles (mineral particles do not adsorb oil under these conditions). Carbon particles coated with very thin layer of oil coalesce and form microagglomerates (8) that can be readily separated from the slurry by, for instance, flotation. The microagglomerates are filtered using, for instance, a vacuum belt filter, rinsed thoroughly with recycled/distilled water, dewatered and utilized as carbon fuel product. The slurry containing mineral particles of about 0.05 mm and larger will be combined with the carbon free slurry containing particles of about 0.05 mm and smaller (FIG. 1). The mineral particles will be separated from liquid phase using, for instance, vacuum belt filter, rinsed thoroughly with hot recycled/distilled water and dewatered thus forming mineral matter product that can be utilized, for instance, as a material for lining electrolysis cells. If required, the dewatered mineral matter product may be subjected to thermal treatment (300-850° C.) resulting in removal of traces of residual organics, destruction of residual free cyanides and calcination of some inorganic components of the mineral matter. Thermally treated mineral matter accounts for about 27 w/w % only of the mass of spent potliner subjected to processing as described in this invention.

[0028] The clarified leachates from all thickeners as well as filtrates collected from the filtering equipment are combined (FIG. 1) and directed to commercial water treatment and distillation system. About 85 w/w % of collected leachates and filtrates will be converted into distilled water that will be recycled for spent potliner processing. The remaining non-distillable, concentrated liquor will contain precipitated sodium fluoride (sodium fluoride solubility is 42,200 mg/L water at 18° C.; it is reduced significantly in the presence of other water soluble compounds like, for example, NaOH, Na₂CO₃), that can be readily separated by filtration. The sodium fluoride product can be recycled for generation of calcium fluoride (CaF₂) or hydrogen fluoride (HF); both are required by aluminum manufacturing process. The concentrated liquor freed of precipitated sodium fluoride and enriched with NaOH (generated by reacting NaF with CaO in aqueous phase) will form the Bayer Liquor product composed mainly of caustic soda (NaOH), aluminates (NaAlO₂) and sodium carbonate (Na₂CO₃). The Bayer Liquor can be recycled within the aluminum manufacturing process for recovery of cryolite in the Bayer plant.

[0029] Having described the foregoing features and advantages of the present invention the following examples are provided by way of illustration, but not by limitation.

EXAMPLE 1

[0030] Oxidative destruction of cyanides in leachates and filtrate from spent potlining material.

[0031] A spent potliner was pulverized to pass 0.355 mm screen and was subsequently processed in a laboratory scale in a 1500 ml stainless steel heated vessel in a step-by-step operation (FIG. 1) carried out according to the present invention.

[0032] To simulate the oxidation of cyanides extracted/leached from the digested spent potliner taking place in a tumbler and extensive extraction and cyanides oxidation system, ambient air was blown into vigorously agitated (300 rpm), high pH aqueous slurry (temp. 95-99° C.) containing pulverized particles of the spent potliner. The leachates and the filtrate obtained after completion of all steps of the extensive extraction and cyanides oxidation (FIG. 2) of the potliner were combined and analyzed for total cyanides. The total volume of combined leachates and filtrate was 6.7 times that of the volume of processed spent potliner. The content of total cyanides in the combined leachates and filtrate was 0.038 mg/L. Based on mass balance for cyanides contents in the untreated potliner, the solid residue after extensive extraction and cyanides oxidation of the spent potliner and the combined leachates and filtrate, it was calculated that 99.5% of total cyanides solubilized in combined leachates and filtrate was destructed under treatment conditions applied according to the present invention.

[0033] The combined leachates and filtrates (FIG. 1) generated by a commercial scale plant of the present invention would be treated in a commercial water treatment and distillation system thus resulting in recovery of about 85 w/w % of combined leachates and filtrates in the form of distilled water for recycling into the process. Based on total cyanides content in the combined leachates and filtrate (0.038 mg/L), the remaining non-distillable liquor would have total cyanides concentration of less than 0.32 mg/L. This liquor, freed of precipitated sodium fluoride, would be recycled to Bayer plant for utilization. The total cyanides content of the liquor (<0.32 mg/L) is comparable to Canadian Environmental Quality Guidelines for Maximum Acceptable Concentration of cyanides (0.2 mg/L) for community water.

EXAMPLE 2

[0034] Quality of carbon product separated from extracted/leached potliner residue.

[0035] Results of analyses of the carbon fuel product separated in a laboratory scale operation from extracted/leached spent potliner, as described in the present invention (FIG. 1), are presented below: Analyses (on dry basis) Carbon Fuel Product Cyanide TCLP leachate 0.113 mg/L Fluoride TCLP leachate 8.75 mg/L Total cyanides concentration 10.3 ppm Total fluoride concentration 1.06 w/w % Total sodium concentration 1.41 w/w % Ash content 20-27 w/w % Heating value ˜26,600 kJ/kg

[0036] The fluoride in carbon fuel product is present mainly in ash, in the form of calcium fluoride (CaF₂). CaF₂ melts at 1423° C., boils at about 2500° C., and has extremely low solubility in water (17 mg/L water at 18° C.). High CaF₂ melting temperature should not have any deleterious effects on ash behavior during combustion. The results of analyses presented above indicate that the carbon separated from extracted/leached potliner residue should be acceptable for co-combustion in cement plants, coal fired power stations and other carbon fuel fired energy producing facilities.

EXAMPLE 3

[0037] Properties of mineral matter product after extracting/leaching and carbon fuel separation.

[0038] The mineral matter product obtained in a laboratory scale operation by extracting/leaching the spent potliner, followed by separation of carbon from extracted leached potliner, as described in the present invention (FIG. 1), has the following properties: Analyses (on dry basis) Mineral matter Product Cyanide TCLP leachate 0.90 mg/L Fluoride TCLP leachate 17.7 mg/L Total cyanides concentration 63.3 ppm Total fluoride concentration 1.81 w/w % Total sodium concentration 2.93 w/w % Weight loss on heating (850° C.) 31.8 w/w %

[0039] The mineral matter product accounts for about 40 w/w % of total spent potliner used for processing. Low fluoride TCLP leachate (17.7 mg/L) indicates that the mineral matter product contains no sodium fluoride. Solubility of calcium fluoride in water (see Example 2) is equivalent to 17 mg/L and agrees very well with fluoride TCLP leachate (17.7 mg/L).

[0040] Thermally treated (at 850° C.) mineral matter product accounts for about 27 w/w % of the total spent potliner processed as described in this invention. Cyanide TCLP leachate and total cyanides concentration in thermally treated (850° C.) mineral matter product are reduced to 0.201 mg/L and 48.0 ppm respectively. Whole Rock Analysis of the thermally treated mineral matter yielded the following results (w/w %) for the metals and other elements analyzed as oxides: Al as Al₂O₃ 23.80 Ba as BaO 0.03 Ca as CaO 7.24 Fe as Fe₂O₃ 12.30 Mg as MgO 4.89 Mn as MnO 0.40 P as P₂O₅ 0.11 K as K₂O 0.55 Si as SiO₂ 17.60 Na as Na₂O 5.29 Sr as SrO 0.03 Ti as TiO 0.55 Zr as ZrO₂ 0.47 Total metals & other elements 73.26

[0041] The properties of mineral matter product and/or the product of its thermal treatment indicate that the products might be recycled for generation of lining for Hall-Heroult electrolysis cells cement manufacturing and other applications.

[0042] References

[0043] 1. J. L. Bernier et al., “The LCLL Process-Spent Potlining Recycling Solution”, 32^(nd) Annual Conference of Metallurgists, Light Metals, Quebec City, Quebec, Canada, Aug. 28-Sep. 2, 1993

[0044] 2. R. P. Pawlek, Journal of Metallurgy, November 1993, page 48

[0045] 3. Federal Register, USA, Vol. 65, No. 134, Wednesday, Jul. 12, 2000

[0046] 4. G. Lever, U.S. Pat. No. 4,816,122, issued Mar. 28, 1989

[0047] 5. R. J. Grolman et al., U.S. Pat. No. 5,470,559, issued Nov. 28, 1995

[0048] 6. J. F. Bush, U.S. Pat. No. 4,889,695, issued Feb. 20, 1985

[0049] J. F. Bush, “Process to Produce AlF₃, Caustic and Graphite from Spent Potlinings in an Environmentally Acceptable Manner”, Light Metals 1986, ed. R. E. Miller, Warrendale, Pa., TMS, 1986, pp. 1081-1099

[0050] F. M. Kimmerle et al., Light Metals 1989, ed. P. G. Cambell, Proceedings of the Minerals, Metals and Materials Society, 1988, pp. 387-394

[0051] F. M. Kimmerle et al., “Extraction and Processing for the Treatment and Minimization of Wastes”, ed. J. Hager, B. Hansen, W. Imrie, J. Pusatori and V. Ramachandran, The Minerals, Metals and Materials Society, 1993

[0052] 7. C. A. Young et al., “Cyanide Remediation: Current and Past Technologies”, Proceedings of the 10^(th) Annual Conference on Hazardous Waste Research, pp. 104-129

[0053] E. T. Gandy et al., “Compatibility of Organic Waste and Cyanide During Treatment by the Extended Aeration Process”, Proceedings of Industrial Waste Conference, Pardue University, Lafayette, Ind., 1981, pp. 484-495

[0054] 8. Y. Briker et al., “Feasibility of Aglofloat Process for Deashing and Desulfurization of High Sulfur Coals”, Processing and Utilization of High Sulfur Coals, IV, ed. P. R. Dugan, D. R. Quigley and Y. A. Attin, 1991 Elsvier Science Publishers B.V., Amsterdam, pp. 357-373

[0055] W. Pawlak et al., “The Cleaning Efficiency of the Aglofloat Process”. 1991 International Conference on Coal Science Proceedings, 16-20 September, University of Newcastle-upon-Tyne, UK, pp. 897-900

[0056] B. L. Ignasiak et al., “Engineering Development of Selective Agglomeration Technology”, ed. W. S. Blaschke, Proceedings of the 12^(th) International Coal Preparation Congress, Cracow, Poland, May 23-27, 1994, pp. 515-520

[0057] W. Pawlak et al., “Development of the Aglofloat Process for Cleaning High Ash and High Sulfur Coals”, Coal Preparation Conference, Kentucky, May 3-5, 1994. 

What is claimed is:
 1. A method for processing fine particles of spent potliner suspended in aqueous slurries containing caustic soda, separating and recovering from said aqueous slurries metallic aluminum, Bayer Liquor, solid sodium fluoride, carbon fuel and mineral matter; the method comprising the steps: a) separating large pieces of metallic aluminum and steel/iron debris from the potlining material by a combination of magnetic separation and manual sorting, crushing so obtained potlining material so the product of crushing passes 100 mm screen; b) tumbling so prescreened potlining material in hot aqueous slurry containing caustic soda, in presence of air, under conditions of autogenous grinding; c) screening the hot slurry exiting the tumbler using 0.5 mm or less trommel screen, washing oversize particles with hot, fresh water, separating residual metallic aluminum from washed oversize particles and recycling the oversize particles for tumbling; d) diluting the slurry that passed the trommel screen with fresh water and agitating the hot fine slurry in a first battery of extracting/leaching mixers supplied, from the bottom with steam and air; e) thickening (in a thickener) the hot fine slurry that exits the first battery, separating the clarified extract/leachate, pumping the thickened slurry and diluting it with fresh water prior to agitating it again in a second battery of extracting/leaching mixers supplied with steam; f) thickening the slurry exiting the second battery, separating the clarified extract/leachate, filtering the thickened slurry using horizontal vacuum belt filter or other filtering device, washing the particles with hot, fresh water and dewatering the particles on the belt filter; g) suspending the dewatered particles in fresh water to form a slurry, agitating and heating so obtained slurry with steam in a third battery of extracting/leaching mixers thus forming primary hot fine slurry; h) thickening the primary hot fine slurry exiting the third battery, separating the clarified extract/leachate, pumping the thickened slurry for gravity/size separation and separating the thickened slurry into two streams: one stream of slurry containing mineral particles with size 0.05 mm and larger and one stream of slurry containing all carbon particles and mineral particles with size 0.05 mm and smaller; i) adding small quantities of suitable mineral oil to the stream of slurry containing all carbon particles and mineral particles with size 0.05 mm and smaller, passing so obtained oil containing stream through a commercial high shear agitator and separating by flotation or other techniques the oil agglomerated carbon particles from the residual slurry containing the mineral particles with size 0.05 mm and smaller; j) filtering the oil agglomerated carbon particles using horizontal vacuum belt filter or other filtering device, washing the agglomerated carbon particles with fresh water and dewatering them on the belt filter thus producing carbon fuel product; k) combining the stream of slurry containing mineral particles with size 0.05 mm and larger with the stream of residual slurry containing the mineral particles with size 0.05 mm and smaller, filtering the combined streams using horizontal vacuum belt filter or other filtering device, washing the mineral particles with hot fresh water and dewatering the particles on the belt filter thus producing mineral matter product; l) combining all streams of produced clarified extracts/leachates and filtrates and processing them in a commercial water treatment and distillation system thus generating distilled water that is recycled for spent potliner processing and non-distillable liquid; m) separating by filtration the non-distillable liquid containing precipitated sodium fluoride (NaF) into solid NaF product and Bayer Liquor product; n) subjecting, if required, the mineral matter product to thermal treatment/calcination within the temperature range of 300-850° C.
 2. A method according to claim 1, where in step a) spent potlining material, freed of steel/iron debris and large pieces (>20 mm) of metallic aluminum is crushed and prescreened to reduce the size of the largest particles of the potlining material to less than 200 mm.
 3. A method according to claim 1, where in step b) crushed and prescreened potlining material, suspended in a hot (50° C. up to boiling point) aqueous slurry containing sodium hydroxide in concentrations 0-10 g/L or 60-200 g/L, is pulverized under autogenous grinding conditions (rpm: 3 and over; solids mass flow: over 1.0 tph; solids concentration in a slurry: about 50-65 w/w %) in a tumbler (equipped with lifters) through which fresh air is flowing at volumetric flow rates of 1-60 m³/1 ton potlining material/min.
 4. A method according to claim 1, where in step c) oversize particles separated on the screen, at the exit from the tumbler and freed of metallic aluminum are subjected to size reduction, prior to recycling them for tumbling.
 5. A method according to claim 1, where in step c) hot slurry after exiting the tumbler is passed through the screen with openings of not less than 0.15 mm and not more than 1.0 mm.
 6. A method according to claim 1, where in step d) hot slurry after exiting the tumbler and passing the 0.15-1.0 mm screen is diluted with fresh water to solids concentration of about 15-30 w/w %.
 7. A method according to claim 1, where in step e) the about 15-30 w/w % solids concentration hot fine slurry, exiting the battery of mixers, is separated in a thickener into two streams: a stream of clarified, solid particles free, extract/leachate and a stream of thickened slurry with about 40-50 w/w % solids concentration.
 8. A method according to claim 1, where in a step h) the stream of thickened slurry with about 40-50 w/w % solids concentration is subjected to separation using spiral separator into two streams: the first stream containing all carbon particles and mineral particles of 0.1 mm and smaller and the second stream containing mineral particles of
 0. 1 mm and larger.
 9. A method according to claim 1, where in step i) suitable mineral oil to be added to the stream of slurry containing carbon particles is defined as any commercial mineral light oil (like Diesel oil, naphta, kerosene, jet fuel or others), or any blend of commercial mineral light oils, or a blend of any commercial mineral light oils with heavy oils/bitumen (API Gravity below 16) where the heavy oil/bitumen component accounts for up to 70 w/w % of the total blend.
 10. A method according to claim 1, where in step i) the oil agglomerated carbon particles are separated using any suitable system (like flotation, cycloning or other methods), from the residual slurry containing mineral particles of 0.05 mm and smaller.
 11. A method according to claim 7, where the thickened slurry with about 40-50 w/w % solids concentration is either diluted with fresh water to about 15-30 w/w % solids concentration and treated in a battery of mixers, or is subjected to filtration and the solid particles deposited on a filter are thoroughly washed with hot (50-95° C.) fresh water, dewatered and suspended in fresh water to form slurry with about 15-30 w/w % solids concentration that is agitated in a battery of mixers.
 12. A method according to claim 9, where suitable light mineral oil, or a blend of light mineral oils, or a blend of light mineral oils with heavy oil/bitumen to be added to the stream of slurry containing carbon particles is, prior to addition, emulsified with water using any commercially available emulsification system.
 13. A method according to claim 9, where quantities of suitable light mineral oil, or a blend of light mineral oils, or a blend of light mineral oils with heavy oil/bitumen to be added to the stream of slurry containing carbon particles are in the range of 0.1-10.0 w/w % based on mass of carbon particles in the slurry stream.
 14. A method according to claim 11, where the slurry diluted to about 15-30 w/w % solids concentration is agitated at such agitation intensity that the concentration of solid particles in the slurry, at any location in the mixer, is essentially the same.
 15. A method according to claim 11, where the slurry diluted to about 15-30 w/w % solids concentration is extracted/leached in a battery of mixers (one to five mixers per battery), where each mixer is heated with steam to keep the temperature of the slurry from 50° C. up to its boiling point.
 16. A method according to claim 15, where the slurry is also supplied with air at volumetric flow rates from 0.5-5.0 m³/mixer/min. 